Tensile enamel

The first commercial applications of polypyromellitimides were as wire enamels, as insulating varnishes and for coating glass-cloth (Pyre.ML, Du Pont). In film form (Kapton) many of the outstanding properties of the polymer may be more fully utilised. These include excellent electrical properties, solvent resistance, flame resistance, outstanding abrasion resistance and exceptional heat resistance. After 1000 hours exposure to air at 300°C the polymer retained 90% of its tensile strength. 

Thornton, J. B., Retief, D. H. Bradley, E. L. (1986). Fluoride release from and tensile bond strength of Ketac-Fil and Ketac-Silver to enamel and dentine. Dental Materials, 2, 241-5. 

Some other physical properties of enamel glass are density, from 2.5-3 5 g/mL Mohs hardness. 5-6 tensile strength. 3d-103 MPa (4.900-15.000 psi) compressive strength, 1380-2760 MPa(2-4x It)5 psil, modulus of elasticity. 55-83 GPa (8- 12x I06 psi) and dielectric constant, 5-10. 

Polyimide films have excellent electrical properties and a tensile modulus of over 400,000 psi at 25°C. Over 60 percent of this modulis is retained at 200°C. Polyimide wire enamels are stable for up to 100 Oiousand hours at 200°C. Polyimide fibers have a tenacity of 7 g/denier at 25°C and over 1000 hrs at 2833C is required to reduce the value to 1 g/denier. 

Adhesion of fired enamel to the metal base is assessed practically by impact tests (e.g. by a falling steel ball). If the adhesion is poor, tfie enamel will peel off on impact, showing a clean metal surface. More precise determinations are based on tensile tests of metal specimens joined with an enamel layer. Very good adhesion is then indicated by failure in the enamel and not in the contact surface. 

Industrially, enameling is currently mainly carried out on steel (and steel alloys). The applied layers in steel-enameling adhere very firmly to the substrate as a result of chemical reactions which take place between the glowing metal and the molten frits, leading to a composite material. The sheet steel-enamel bond can withstand a tensile. stress of 1000 kp/cm. In addition to steel (ca. 90%) ca.st iron (ca. 6%) and aluminum (ca. 4%) are also enameled. Annually ca. 450 10 t/a of enamel is applied to a total surface of ca. 750 10 m. Ca. 30% is used in Europe and 20% in the USA. In the Federal Republic of Germany there are ca. 50 enameling plants with an annual enamel consumption of ca. 13 lO- t. 

Although these contacts are transient, they do result in distinct forces on the tooth. The tooth is known to undergo tiny lateral displacements when loaded and this results in compressive stress on the side towards which the tooth bends and tensile stress on the other side [82,85], These stresses lead to miao-fractures in the enamel or dentine... 

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